Carbonate landforms occupy 20% of the world's land area, and due to their strong solubility, the soil layer is shallow, the underlying surface karst zone is full of fissures and pipeline networks, and the spatial heterogeneity is high, and the hydrological process is extremely complex and easily affected by the external environment. As global change intensifies, how hydrological processes in karst respond to climate extremes and human disturbances is critical. In order to accurately characterize the hydrological and material migration and transformation processes based on limited parameter conditions under the extremely high heterogeneous geological background of karst areas, the key is to study the mutual feedback mechanism between hydrological processes, vegetation, soil, surface karst zone, bedrock and other factors.
Chen Hongsong's research group from the Institute of Agroecology of the Chinese Academy of Sciences found that the soil thickness was significantly positively correlated with the depth of the surface karst zone at the scale of small watersheds, and there was a good spatial correspondence between vegetation communities, soil types and karst development degrees in small watersheds, but the quantitative relationship between rock and soil structure and hydrological function of karst slopes was still unclear.
In view of the above scientific problems, relying on the three-dimensional water and soil process observation platform of the key zone of karst slope of Huanjiang Station, Chen Hongsong's research group comprehensively applied the methods of full-slope geotechnical trench excavation, quantitative characterization of geotechnical structure, and dynamic monitoring of multi-interface hydrological and hydrochemical processes, and monitored a total of 158 different interfaces (surface runoff, Rainfall-runoff events, and the structural parameters of soil-surface karst zone were investigated (924 groups of paired data of soil thickness and surface karst weathering degree). The results show that: (1) The soil thickness increases with the increase of the weathering degree of the underlying surface karst zone, and there is a significant positive correlation between the soil thickness and the thickness of the surface karst zone (P<0001）；(2) Under the condition of non-extreme rainfall (rainfall < 50 mm for 12 h), the surface runoff was significantly positively correlated with soil thickness (P<0.).001), there was a significant negative correlation between soil flow and soil thickness at the rock-soil interface (P=0.).0018), the critical soil thickness of the runoff path in the key zone of karst slope is 50cm;(3) Under the condition of 50 mm and 12 h of rainfall, the coexistence of surface runoff and rock-soil interface soil mid-flow occurred in different thickness areas, and the surface runoff was significantly positively correlated with the rock-soil interface soil mid-current, indicating that there was obvious hydrological connection between surface hydrological process and subsurface hydrological process. (4) Compared with shallow soil, the development of alluvial layer in deep soil, and the higher water storage capacity and CO2 concentration promoted the development of surface karst zone, while the weak permeable alluvial layer and high permeable surface karst zone led to the increase of surface runoff and the decrease of middle flow in the rock-soil interface soil. This is the essential reason for the co-evolution of soil-surface karst zone structure and hydrological processes in the key karst zone. The double 50 threshold (50 cm soil thickness and 50 mm rainfall) and the co-evolution law of structure-process-function in the key karst zone can provide theoretical support for the sustainable and efficient utilization of water and soil resources and the construction of high-precision water and soil process models in karst areas.
The above results were published in the Journal of Hydrology, a top journal in SCI Zone 1 (IF=6.) with the title of Co-evolution among Soil Thickness, Epikarst Weathering Degree, and Runoff Characteristics on a Subtropical Karst Hillslope4), this research was supported by the Guangxi Key R&D Project (AB22035058) and the National Key R&D Project (2022YFF1300702).
Fig.1 Relationship between soil thickness and weathering degree of surface karst zone.
Fig.2 Relationship between runoff and soil thickness under normal rainfall conditions (rainfall < 50 mm for 12 h).
Fig.3 Schematic diagram of the co-evolution mechanism of soil-surface karst zone structure and hydrological processes on karst slopes.